Document FR 2 889 089 describes an example of a wall element of the above-specified type forming part of the wall of a hollow blade for an airplane turbojet turbine. FIG. 1 shows another known example, analogous to that of FR 2 889 089. In this example the shape of the diffusion portion 20 is determined by aerodynamic and thermal constraints set by the designer. However, this diffusion portion 20 penetrates very deeply into the wall 22. This has the effect of locally thinning the wall 22 and of weakening it from a mechanical point of view. Under strong and repeated thermal stresses, this thinning encourages the local appearance of cracking, and in the long term of cracks that propagate throughout the part. In particular, is at the hole 23, where the thickness E′ of the wall is at its thinnest, that cracks or crack starters are generally seen to appear.